1. Field of Invention
The present invention relates to the fabrication of tools used in the assembly and interconnection of semiconductor devices and associated structures. More particularly the present invention relates to the fabrication of miniaturized semiconductor wire bonding tools such as ceramic capillaries, tungsten carbide bonding wedges, die-collets, and the like, with reduced dimensions and improved dimensional accuracy well beyond the capability of prior art manufacturing techniques.
2. Description of Prior Art
The universal trend, in the electronics, aerospace, medical and other fields of industry, toward smaller, more complex, lighter, more tightly toleranced, and more highly integrated products to be used in more aggressive, corrosive, hotter or constrained environments, creates a situation which has pushed current manufacturing technologies to their limits of capability.
Nowhere is the need for miniaturization and integration felt more intensely than in the microelectronics industry. For example, the market's craving for lighter, smaller and lower power electronic appliances such as laptop computers, palm-sized Personal Digital Assistants (PDAs), lightweight cellular phones and many other sophisticated electronic gadgets, has driven designers of such products into the so-called ‘system-on-a-chip’ (SOC), ‘chip-scale-package’ (CSP) and multilevel package design.
As a result, in the fabrication of integrated circuits (ICs) or semiconductor chips—which pervade every field of life and industry today—there is enormous impetus to increase the functionality of silicon chips, which in turn implies increasing the density of electronic circuits on a single chip. For the past forty years, the number of transistors on a chip has—rather remarkably—followed Moore's Law, which states that the number of transistors on a chip is bound to double every 18 months. In Intel's latest Micro2000 chip, that number already exceeds 10 million.
As silicon technology progresses towards higher integration, there is a corresponding increase in the I/O (input/output) count, also termed lead or wire count, i.e. the number of interconnections between the silicon chip and the external circuitry. The world wide lead count for IC interconnections is expected to surpass 9 trillion by 2003. The current average wire count per IC is approximately 45 while the high end of wire counts is approximately 500 for ASIC devices, graphic chips, and chips that support processors.
As a result, increasing demands are being placed on the interconnect or bond pitch, i.e. the distance between contiguous interconnections. The smaller the bond pitch, the less die (or chip) real estate is taken up by the bond pad, the area on the silicon chip specifically reserved for the interconnection or bond. If bond pads can be reduced in size and brought closer together, the die can be shrunk, resulting in a larger number of dice and significantly higher revenue per wafer. With devices tending towards very high integration, the bond pad pitch has to decrease.
The major bonding process used for semiconductor interconnection, accounting for more than 90 percent of the global bonded lead count, is wire bonding. In the simplest terms, wire bonding is a low temperature welding process whereby a thin conductive wire, of the order of 25 μm in diameter and generally of gold, aluminum, copper or various metal alloys, is connected from the bonding pad on the IC to the corresponding bonding area of the microelectronic package's lead frame.
The process consists of bringing the bonding wire in close proximity with the bond site and applying ultrasonic energy which increases the dislocation density in the area where the wire and bond site are contiguous, resulting in lower flow stress and modulus of elasticity and increased rate of diffusion. As material flow occurs, microscopic slip planes shear across each other, generating new metallurgically clean surfaces which diffusion weld to each other. Higher ultrasonic frequencies increase the strain rate, resulting in a more efficient energy transfer from the wire to the bond interface.
There are currently two wire bonding techniques: ball bonding and wedge bonding. Ball bonding is a high-speed omnidirectional process, performed almost exclusively with gold wire, which lays claim to more than 95 percent of the total wire bonded lead count. Gold ball bonding is usually performed using tiny ceramic tubelets called capillaries.
The first step in the ball bonding cycle consists of paying out a predetermined length of gold wire through the capillary orifice or tip, upon which the end of the protruding wire is melted in an electric arc (electronic flame-off, EFO) to form the so-called free air ball. The capillary then positions the ball onto the IC bond pad while ultrasonic energy is applied, thus generating the first or ball bond. Next, the capillary retracts while bonding wire is paid out through the capillary orifice. The capillary then proceeds toward the corresponding bond site on the lead frame. The motion and path of the capillary are carefully engineered so as to generate a precise wire loop geometry. The second or tail bond is then created, again using ultrasonic energy, upon which the capillary tip sections off the bonding wire after the second bond and the next cycle starts.
Wedge bonding is a similar but lower speed process using aluminum, gold, and sometimes copper wire. No ball is formed in the process. Wedge bonding is capable of finer pitch than ball bonding and is usually performed using wedge shaped tungsten carbide tools called bonding wedges.
Prior art bonding tools have been produced from a variety of materials such as glass, tungsten carbide, titanium carbide, steel alloys, alumina, zirconia, monocrystalline ruby, etc., and with many different geometries. For example, prior art wire bonding tool tips have included circular, V-shaped, square or rectangular tips with concave or flat contact areas doted with parallel or non-parallel grooves, patterns or textured surfaces to promote bonding.
During the bonding process, the bonding tool's tip is subjected to heat, abrasion and stress. Abrasion and tool wear ultimately change the surface texture of the tip, affecting the transfer of ultrasonic energy to the bond interface, while repeated cycling of ultrasonic energy through the bonding tool may result in microcracks, culminating in fatigue-induced fracture.
Hence, prior art efforts have focused on improving the mechanical properties and abrasion resistance of wire bonding tools. For instance Dobbs, et al., U.S. Pat. No. 4,667.867 and Elwood, et al. U.S. Pat. No. 5,217,154 propose equipping bonding tools with tips made from hard materials. Others have proposed hard tip coatings. To obviate the deleterious effects of ultrasonic cycling, other inventors suggest making the bonding tool shank or main body from a stiffer material in order to reduce the amplitude of the displacement caused by the ultrasonic vibration.
Yet, when it comes to solving the problems related to finer bonding pitch and its corollary, smaller bond pads, the prior art has only been marginally successful.
The reason for this is because reducing bond pad area and pitch has a domino effect on the entire technology of IC interconnection. The findamental, complex and interrelated issues to be addressed in ultrafine pitch wire bonding include dimensioning the lead frame, resizing bond pad and pitch on the die, determining bonding wire diameter and length as well as the height and profile of the wire loop, bonding process optimization, bonding tool design and various material properties and reliability issues. These issues constitute today's biggest bottleneck in IC design and assembly and, as I/O count grows, that bottleneck gets bigger.
Whereas, from an IC designer's point of view, the minimum bond pad pitch is what should dictate the capillary tip diameter, and the maximum acceptable loop height should dictate the cone angle—or tip diameter—of the capillary, in reality it is the opposite situation that prevails.
When bond pad pitch started to shrink, the first problem capillary designers had to solve was a purely geometrical one: capillary tips had to clear previously produced bonds and wire loops. A makeshift solution was to grind the tips of standard capillaries to a smaller cone angle, resulting in today's so-called “bottleneck capillaries”.
A direct consequence of this geometry is that, since the capillary borehole diameter remains constant, the capillary tip wall thickness, and subsequently its mechanical strength, are considerably weakened, hence bottleneck capillaries are much more fragile. Also it is very difficult to precisely CNC grind the typically 25 μm thin brittle walls of state-of-the-art ceramic bottleneck capillary tips. Automated grinding lines can, at best, achieve machining tolerances of approximately 2.5 μm, which is insufficient when capillary design features are further reduced in size. Furthermore, stresses and micro-cracks, inevitably introduced during the grinding operation, combined with the thermal and ultrasonic cycling inherent to the bonding process, increase the probability of in-service capillary tip fracture, a dreaded prospect for any semiconductor firm, as bonding is the final step before IC encapsulation. As a result, bottleneck capillaries cost typically 50–100% more than standard capillaries. A partial solution to the weak tip wall problem is to make the capillaries from ZTA (zirconia toughened alumina) but this raises their cost even more.
But it is in attempting to solve the functional problems generated by reduced bond pad size and pitch, that the prior art has failed altogether. Smaller bond pads inherently imply smaller ball bonds—which must be ‘100% on pad’, i.e. the bonded area should, ideally, correspond to the greatest circle inscribable in the square bond pad. A smaller ball bond requires a smaller free air ball. Consequently, in fine-pitch bonding, consistent production of a small free air ball is an absolute necessity.
Free air ball size is usually expressed as BSR—the ratio of the free air ball diameter to that of the wire—and is governed by factors such as the wire composition, diameter and tail shape after the second bond, the EFO conditions and capillary geometry. State-of-the-art BSRs are typically above 2. The industry is trying to bring this down to the 1.4–1.6 range or even lower.
After the second bond, the bent and deformed end of the wire must be totally consumed by the EFO arc. Clearly a smaller tail volume will yield a smaller BSR. Wire tail shape and volume depend on the wire diameter and the capillary's inner chamfer. Hence, a smaller free air ball dictates a smaller chamfer as well as a smaller wire diameter, below the standard 25 μm diameter. However, the diameter of the capillary borehole must be proportionate to the diameter of the wire. If the hole is too tight, wire drag will be too high, and if too large, the looping profile, loop consistency and wire bond positioning are significantly affected.
In summary, to allow further reduction in bond pad pitch, it is imperative that capillaries having smaller tips and boreholes be available. The current prior art is unable to comply with this requirement and IC designers are, therefore, reduced to using the smallest capillaries they can find on the market.
From the explanations provided above, it will have become obvious to anyone skilled in the art, that state-of-the-art semiconductor wire bonding tools, even bottleneck capillaries, constitute the true bottleneck in very high IC integration.